A method of growing semiconductor heterostructures based on gallium nitride

ABSTRACT

The method of growing non-polar epitaxial heterostructures for light-emitting diodes producing white emission and lasers, on the basis of compounds and alloys in AlGaInN system, comprising the step of vapor-phase deposition of one or multiple heterostructures layers described by the formula Al x Ga 1-x N (0&lt;x≦1), wherein the step of growing A 3 N structures using (a)-langasite (La 3 Ga 5 SiO 14 ) substrates is applied for the purposes of reducing the density of defects and mechanical stresses in heterostructures.

1. FIELD OF THE INVENTION

The invention is related to methods of manufacturing of semiconductormaterials and devices, and more particularly, to manufacturing non-polarepitaxial heterostructures of third group elements nitrides (further A³Nstructures) by Organometallic Vapor—Phase Epitaxy (further OMVPE) whichare usually used for such devices, as lasers, light emitting diodes(LEDs), and particularly, white LEDs.

2. DESCRIPTION OF THE RELATED ART

A³N semiconductor heterostructures are basic materials for design andmanufacture of high efficient light emitting diodes and lasers invisible and ultraviolet parts of optical spectrum of radiation,including white LEDs.

In the reference [1] use of converting dark blue and/or ultra-violetradiation of GaN-mis structures into longer wavelength radiation invisible part of spectrum with the help of covering these structures bystocks phosphors was offered for the first time.

In the reference [2] design of white light emitting diodes on the basisof dark blue p-n AlGaInN heterostructure emitters covered byYttrium-Aluminum-Garnet phosphor has been offered. Part of the primarydark blue radiation of emitters is converted into yellow radiation ofphosphor. As a result, mixing of blue radiation from an emitter andcomplementary yellow luminescence exited by the blue radiation inphosphor produce white light by LEDs with certain coordinates ofchromaticity.

Three basic designs of white light-emitting diodes essentially differingfrom each other are known:

-   -   light-emitting diodes on the basis of an emitter of dark blue        color of luminescence which is covered by a layer of stocks        phosphor converting a part of dark blue radiation into yellow        radiation;    -   light-emitting diodes on the basis of an emitter of ultraviolet        radiation which is covered by a layer of stocks phosphor        converting ultraviolet radiation into red, green and dark blue        bands of luminescence (RGB system);    -   full-color light-emitting diodes containing three separate        emitters radiating in red, green and dark blue parts of spectrum        (RGB system).

Despite of distinction, improvement of parameters of all listed types ofwhite light-emitting diodes demands perfection of methods of epitaxialA³N-heterostructures growth and increase of quantum output of radiationof phosphors.

For mass production of light emitting diodes the most preferable methodof manufacturing A³N-heterostructures is the method of OrganometallicVapor—Phase Epitaxy (OMVPE).

Sapphire (Al₂O₃), silicon carbide (6H—SiC), gallium nitride (GaN) andaluminum nitride (AlN) are used as substrates for A³N epitaxialstructures growth. Cheaper sapphire substrates are most of all used.Silicon carbide substrates in some times more expensive than sapphireones and, therefore, are used not so often. Close to ideal there couldbe substrates made of GaN or AlN, but their mass production is notachieved yet.

Typical A³N-heterostructures for light-emitting diodes contain followingfunctional parts:

-   -   a single crystal substrate of sapphire or silicon carbide which        surface is crystallographic c-plane (0001) defining        crystallographic type of A³N epitaxial layers, for example,        wurtzite type of their crystal structures and azimuthally        orientation of crystallographic lattices;    -   wide-bandgap emitters, as a rule, n-type and p-type        Al_(X)Ga_(1-X)N layers providing effective injection of        electrons and holes and their confinement in active region of        the heterostructure;    -   an active region containing, as a rule, a set of narrow-bandgap        layers of such materials, as In_(X)Ga_(1-X)N alloys which are        usually not specially doped;    -   contact epitaxial GaN layers of n-type and p-type conductivity        providing low specific resistance of ohmic contacts and uniform        distribution of current density in a cross-section of a device.

In A³N-epitaxial heterostructures used in various devices, in particularin light-emitting diodes and lasers, density of defects (dislocations,defects of packing, etc.) and also a level of mechanical stresses shouldbe as low, as possible. For example, GaAs laser heterostructures usuallyhave dislocation density not exceeding values of 10²-10³ cm⁻².

In A³N-heterostructures basically exists two sources of defects, firstof which concerns to a difference of lattice parameters of a substrateand A³N epitaxial layers and second one concerns to a mismatch oflattice parameters of layers inside of a heterostructure, for example,between GaN and Al_(X)Ga_(1-X)N layers or between GaN andIn_(X)Ga_(1-X)N layers. In the case of GaN or AlN substrates thecontribution of the first defects source is decreasing and is comparablewith the second defects source contribution.

A³N single-crystal epitaxial layers which have wurtzite type of crystalstructure: AlN (lattice parameter a=0.311 nm), GaN (a=0.316 nm) and InN(a=0.354 nm), grown on single-crystal Al₂O₃ substrates oriented in(0001)-plane (the oxygen sublattice parameter a=0.275 nm) or on 6H—SiCsubstrates (a=0.308 nm), always contain high density of defects,basically dislocations.

Dislocations are formed in interface “substrate epitaxial layer” becausethere is an essential difference of lattice parameters of a substrateand a epitaxial layer. Lattice parameters of epitaxial layers are largerthan a lattice parameter of a substrate (discrepancy up to 16%) anddislocations will spread through heterostructure layers. In typicalAlGaInN heterostructures used in blue and green light-emitting diodes,which have been grown on sapphire substrates, dislocation densities mayhave values 10⁸-10¹⁰ cm⁻². For similar heterostructures grown on SiCsubstrates dislocation densities may have values 10⁷10⁹ cm⁻². Thus, thecontribution of the first source of defects is defined by a value10⁷-10⁹ cm⁻², the contribution of the second source of dislocationsformation inside a heterostructure is equal to 10⁶-10⁷ cm⁻². Inparticular, formation of high density of dislocations and even crackingAlGaN layers is caused by a difference of lattice parameters of GaN andAlN layers (discrepancy of 3.5%) and by their differences in thermalexpansion coefficient values.

For the partial solution of these problems can be used methods. In firstof them before growing a AlGaN layer, for example, n-type emitter layer,a thin In_(0.1)Ga_(0.9)N layer is grown (thickness about 0.1 microns) toprevent cracking a subsequent Al_(X)Ga_(1-X)N (x=0.15-0.20) layer. Inthe second method instead of a bulk Al_(X)Ga_(1-X)N n-type emitter layerwith a constant x-value a strained multiquantum superlattice AlGaN/GaNlayer is grown. The thickness of each layer in the superlattice is about0.25 nm.

A very special feature of Organometallic Vapor—Phase Epitaxy forA³N-heterostructures growth is necessity of abrupt changing temperatureof substrates during a technological process. So, at growing a bufferlayer (usually a very thin amorphous GaN or AlN layer) the temperatureof sapphire or silicon carbide substrates is rapidly decreased from1050° C.-1100° C. down to 550° C. and after finishing the amorphous GaNor AlN layer growth the substrate temperature is rapidly increased up tothe temperature of growth of a single crystalline GaN layer (1050° C.).If process of heating substrates with a buffer GaN or AlN layer is slow,it will lead to crystallization of a thin (about 20 nm) GaN layer andsubsequent growing a thick GaN layer leads to formation of a nonplanarfilm which has great number of defects and figures of growth.

Another necessity of change of substrate temperature during growth isrealized at growing In_(x)Ga_(1-X)N layers (at x>0.1) in active regionof the heterostructure. These layers have a tendency to thermaldecomposition at temperatures above 850° C.-870° C. In this case growingIn_(x)Ga_(1-X)N layers is completed at a lower (800° C.-850° C.)temperature. During increasing the substrate temperature up to 1000°C.-1050° C. the process of heterostructure growth should be interruptedby disconnecting submission of metalloorganic Ga, Al and In precursorsto substrates. With the purpose to exclude thermal decomposition ofIn_(X)Ga_(1-X)N layers they are sometimes covered with a thin (˜20 nm)protective Al_(0.2)Ga_(0.8)N layer. This layer has sufficient stabilityto dissociation up to temperatures about 1050° C. Sharp change oftemperature of a substrate with deposited epitaxial layers (exceptduring a GaN or MN buffer GaN layer growing) can lead to additionalformation of defects and cracking grown layers, for example, AlGaNlayers. Thus, it is desirable to have such methods ofA³N-heterostructures growth, in particular structures for super brightlight-emitting diodes, which allow smooth change of growth temperaturesand exclude interruptions of a growth process at In_(X)Ga_(1-X)N layersgrowing. These methods of growth have also to reduce density ofdislocations generated in interfaces of A³N heterostructure layers.Reduction of dislocations penetrating into a (0001) heterostructuregrown on sapphire or silicon carbide substrates can be achieved by useof special techniques including lateral epitaxial overgrowth(LEO-technology). At first, in this technology a thin buffer GaN layeris usually grown at a low temperature. Then a SiO₂ or Si₃N₄ film isdeposited on the structure surface. In this film narrow long paralleleach other windows are etched down to the buffer layer and then, duringthe next epitaxy process, a thick GaN layer has been grown on SiO₂ orSi₃N₄ film surface at a high temperature. In the same process a A³Nheterostructure is also grown up. It is easy to see, that theLEO-technology is much more complex and more labour-consuming, thanusual technology.

Theoretical and, partially, experimental investigations predictadvantage of use non-polar a-plane (further a-A³N) heterostructures in alot of devices, in particular, in light-emitting diodes and lasers. Incomparison with usual polar heterostructures grown along the polarc-direction [0001] in a-A³N non-polar heterostructures strongelectrostatic fields along the direction of growth are absent. Owing toit, spatial separation of injected electrons and holes in the activeregion of non-polar a-A3N heterostructures is eliminated and, asconsequence, increase of internal quantum efficiencies of radiation inlight-emitting diodes and lasers made on their basis can be expected.

A lot of publications is devoted to growth of a-A³N non-polarheterostructures. In the patent application [3] growth of a-GaN (1120)films on r-plane (1102) sapphire substrates is described. In thepublication [4] advanced a-A³N non-polar heterostructures grown on a-GaNsubstrates are proposed by Sh. Nakamura.

At last, in the patent application [3] the opportunities of a-A³Nnon-polar heterostructures growth on silicon carbide, silicon, zincoxide, lithium aluminates, lithium niobate and germanium substrates arementioned.

Thus, a-A³N non-polar heterostructures growth providing low dislocationsand structural defects densities is rather actual direction oftechnology developments to solve problems of increasing internal quantumefficiencies of light-emitting diodes and lasers and their life-times.

THE BRIEF DESCRIPTION OF THE INVENTION

The subject of this invention is a new method of growing non-polar a-A³Nepitaxial homo- and/or heterostructures on the basis compounds andalloys in AlInGaN system on which have low dislocations and structuraldefects densities in layers on LANGASITE (a-La₃Ga₅SiO₁₄) substratesinstead substrates made of other known materials to use theseA³N-structures in design and manufacturing light-emitting diodes andlasers. The properties of A³N materials and langasite are presented inTable 1.

According to the first aspect of the invention a method of growth inwhich for decreasing dislocations density at the interface “the firstepitaxial Al_(X)Ga_(1-X)N layer—the substrate” and in other functionallayers of light-emitting heterostructure a-langasite substrate is used.Mismatch of c-lattice parameters of the substrate and the firstepitaxial Al_(X)Ga_(1-X)N layer is no more, than within the limits from−2.3% at x=1 up to +1.7% at x=0, and mismatch of their thermal expansioncoefficients in the direction along the c-axis is no more, than withinthe limits from +46% at x=1 up to-15% at x=0. Thus, there are particularx-values at which mismatch of c-lattice parameters of the substrate andthe first epitaxial Al_(X)Ga_(1-X)N layer and mismatch of their thermalexpansion coefficients in the direction along the c-axis are absent(Table 1).

In conformity with the second aspect of the invention, for manufacturinga “white color heterostructure with built-in phosphor” the langasitesubstrate is doped by special impurities to convert part of the primarydark blue radiation of the A³N heterostructure (λ_(MAX)=455 nm) intoyellow radiation of the substrate, thus the substrate structurecorresponds to formula La_(3-x-y)Ce_(x)Pr_(y)Ga₅SiO₁₄.

According to the third aspect of the invention, a topology of thelangasite substrate and a design of the emitter chip are offered, atthat all dark blue radiation of heterostructure is directed into thesubstrate to increase radiation power and to achieve uniform spatialdistribution of color temperature of white radiation.

TABLE 1 Physical A³N type nitrides Langasite properties AlN GaN InNAl_(0.44)Ga_(0.56)N La₃Ga₅SiO₁₄ Crystal wurtzite wurtzite wurtzitewurtzite Trigonal structure group P321 Lattice 3.112 3.189 3.548 3.1558.173 constant a, Å (direction perpendicular to c- axis) Lattice 4.9825.185 5.760 5.099 5.099 constant c, Å (direction parallel to c-axis)Ratio of lattice 0.977 1.017 1.130 1.00 — constants (−2.3%) (+1.7%)(+13%)  (0%) c_(A3N)/c_(La3Ga5SiO14) Thermal expansion 5.3 × 10⁻⁶ 3.17 ×10⁻⁶ 3.0 × 10⁻⁶ 4.11 × 10⁻⁶ 3.56 × 10⁻⁶ (Δc/c), K⁻¹ (direction parallelto c-axis) Thermal expansion 4.2 × 10⁻⁶ 5.59 × 10⁻⁶ 4.0 × 10⁻⁶ 4.98 ×10⁻⁶ 5.11 × 10⁻⁶ (Δa/a), K⁻¹ (direction perpendicular to c- axis) Ratioof thermal 1.49 0.89 0.84 1.15 — expansion coefficients  (+49%)  (−11%)(−16%) (+15%) (Δc/c)_(A3N)/ (Δc/c)_(La3Ga5SiO14) (direction parallel toc-axis)

THE BRIEF DESCRIPTION OF DRAWINGS

The drawings included in this application provide detailed descriptionof advantages of the invention and help to understand its essence.Similar reference numbers represent corresponding parts throughout.

FIG. 1 is a drawing of a polar light-emitting A³N-heterostructure grownby a usual method of epitaxy prototype [2].

FIG. 2 is a drawing of a non-polar light-emitting A³N-heterostructuregrown on a langasit substrate.

FIG. 3 is a schematic view of a light-emitting heterostructure on alangasite substrate with an additional Ce- and Pr-doped langasite layergrown on the surface of the A³N-heterostructure.

FIG. 4 represents an emission spectrum produced by the light-emittingdiode on the Ce- and Pr-doped langasite substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described below with references to drawings.

FIG. 1 represents a typical light-emitting diode heterostructure andchanging bandgap energy in heterostructure layers corresponding toprototypes; U.S. Pat. No. 5,290,393 March/1994, Nakamura; U.S. Pat. No.5,993,542 November/1999, Yanashima; U.S. Pat. No. 5,909,036 June/1999Tanakana. This heterostructure contains an additional n-In_(x)Ga_(1-X)Nlayer (4) grown to prevent cracking a following n-AlGaN (5) emitterlayer which is grown before a multiple quantum wellsIn_(x)Ga_(1-X)N/In_(y)Ga_(1-Y)N active layer (6).

FIG. 2 represents a light-emitting diode heterostructure, grown on alangasit substrate. A profile of changing bandgap energy in differentheterostructure layers is also shown. Unlike the structure representedin FIG. 1 in the offered structure the n-In_(X)Ga_(1-X)N layer (4) andthe p-GaN layer (8) are not grown. The p-GaN layer (8) is a wave guidinglayer which is most effectively used in laser diodes, not inlight-emitting diodes. For growth of a light-emitting diodeheterostructure a langasit substrate (1) having the a-plane orientationand perfect surface treatment (Ra<0.5 nm) is loaded into a reactor of anOMVPE apparatus in very clean nitrogen atmosphere conditions. Afterblowing through the reactor by pure nitrogen hydrogen pressure in thereactor decreases to an operating level nearby 70 Torr. Then thegraphite susceptor with the substrate are heated up to 1050° C. Afterheating during 15 min at hydrogen flow rate of 15 litre/min ammonia withflow rate of 5 litre/min is supplied into the reactor. In this conditionthe process is sustained for 5 minutes. After that high-frequencyheating power is decreased and within 6 minutes the temperature of thesusceptor is stabilized at the level 530° C.

Then, to grow up a GaN buffer layer (2) trimethylgallium (TMG), as thesource gas, with flow rate of 4*10⁻⁵ mol/min is supplied throughseparate injection nozzle into the reactor for 50 seconds. As a result,the GaN buffer layer with thickness of 15 nm is grown. After that, thesusceptor temperature is very rapidly risen up to 1030° C. and TMG withsilane (SiH₄) used as a donor impurity source is supplied into thereactor with flow rate of 7*10⁻⁵ mol/min. The TMG+SiH₄ gas mixture hasflow rate of experimentally selected value to have a doping level of theGaN layer about 2*10¹⁸ cm⁻³. The GaN layer (3) with thickness about 3.2microns grows for 35 minutes. Then the trimethylaluminum (TMAl) issupplied as a source gas, and its flow rate linearly increases from 0 to1*10⁻⁵ mol/min during 5 minutes. As a result, the n-Al_(x)Ga_(1-x)N(x<0.15) (5) layer with thickness of 0.5 microns and with a gradient ofaluminum content is grown. After that, supplying TMG, TMA1 and SiH₄ isstopped, the susceptor temperature has been very rapidly reduced down to860° C. during 5 minutes. Now, submission of TMG and trimethylindium(TMI) is switched on and growth of In_(x)Ga_(1-x)N/In_(y)Ga_(1-y)Nlayers (6) forming a multiply quantum wells structure occurs byperiodically switching TMI flow rates between 7*10⁻⁶ mol/min and 3*10⁻⁵mol/min. Duration of TMI submission with the higher flow rate takes of 3seconds and with the lower flow rate of 16 seconds. Then the susceptortemperature rises up to 1030° C. during 5 minutes and TMG+TMAL flows aresupplied into the reactor again. During growth of AlGaN (9) and GaN (10)layers bis(cyclopentadienyl)magnesium (Cp₂Mg) as a source of acceptorimpurity is supplied into the reactor. The Cp₂Mg flow rate must be highenough to obtain the acceptor concentration of the order 3*10¹⁸ cm⁻³ forproviding low specific resistance of the p-GaN contact layer (10).

In FIG. 3 a design of an emitter for an white light-emitting diode isrepresented. The emitter consists of a heterostructure radiating in darkblue part of spectrum whose layers (2)-(10), according to the invention,are grown on a-langasite substrate by selective OMVPE epitaxy. Thelangasite composition is described by formulaLa_(3-x-y)Ce_(x)Pr_(y)Ga₅SiO₁₄. There are specially prepared recesses inthe substrate for selective heterostructure epitaxy. Before the finaloperation of separating a wafer into chips there are made a number oftechnological operations: photolithography, removal of layers (6), (9)and (10) from part of the selectively grown heterostructure by etching,deposition of the reflecting coating (11) consisting of thin layers ofnickel and gold, and deposition of the ohmic contact (12) layerconsisting of the tin-gold alloy which is needed for the subsequentmounting the emitter on the base of a light-emitting diode. Absorptionof the dark blue radiation of the heterostructure excites yellowphotoluminescence in the substrate, caused by presence of Ce and Pr inlangasite. Effective transformation of part of dark blue radiation intoyellow is provided with absence of air interlayer between theselectively grown heterostructure and langasite surrounding it from theall directions. As a result, due to mixture of dark blue and yellowradiation the emitter generates white light.

In FIG. 4 a typical design of a white light-emitting diode (prototype)is represented in which a dark blue color emitter (13) is used coveredby usual Yttrium-Aluminum-Garnet phosphor (14).

INDUSTRIAL APPLICABILITY

A³N-heterostructures on a-plane langasite substrates grown by the methodproposed in the invention have lower density of defects than structuresby usual methods and have no microcracks. The dislocation density inheterostructures represented in the FIG. 2 may have values less than5*10⁷ cm⁻². Emitters have white color of light with chromaticcoordinates X=0.31, Y=0.31.

[1] SU No 635813, 7 Aug. 1978.

[2] U.S. Pat. No. 5,998,925, 7 Dec. 1999.[3] M. Craven et al, Dislocation reduction in non-polar gallium nitridethin films, PCT/US03/11177, 15 Apr. 2003.[4] Sh. Nakamura, Growth and device strategies for AlGaN-based UVemitters, UCSB, 2004.

1. A method of growing non-polar epitaxial heterostructures forlight-emitting diodes producing white emission and lasers, on the basisof compounds and alloys in AlGaInN system, comprising the step ofvapor-phase deposition of one or multiple heterostructures layersdescribed by the formula Al_(x)Ga_(1-x)N (0<x≦1), wherein the step ofgrowing A³N structures using (a)-langasite (La₃Ga₅SiO₁₄) substrates isapplied for the purposes of reducing the density of defects andmechanical stresses in heterostructures.
 2. The method of claim 1,wherein as substrates for growing A³N structuresLa_(3-x-y)Ce_(x)Pr_(y)Ga₅SiO₁₄ (x=0.1÷3%, y=0.01÷1%) langasitesubstrates are used, that allows to transform a part of dark blueradiation of the heterostructure into a yellow photoluminescence of thesubstrate.
 3. The method of claim 1, wherein as substrates for growingnon-polar A³N structures for monochromatic green and ultravioletradiation devices a-plane langasite substrates are used.
 4. The methodof claim 1, wherein as substrates for growing non-polar A³N structuresfor transformation of ultraviolet radiation into visible radiation,including white light, a-plane langasite (La₃Ga₅SiO₁₄) substrates dopedby suitable phosphors are used.
 5. The method of claim 1, wherein thethickness of the langasite substrate does not exceed 80 microns.
 6. Themethod of claim 1, wherein Ce-doped and Pr-doped langasite buffer layersdeposited on the materials of the group comprising any of Si, Al₂O₃, Geor similar materials, are used as substrates.
 7. The method of claim 1,wherein the step of growing A³N structures is followed by the step ofgrowing an additional phosphor langasite layer on the surface of A³N. 8.The method of claim 1, wherein the thickness of the grown langasitelayer does not exceed 3 microns.